Controlled radical polymerization processes have received much attention over the past decade, because of the possibility to prepare new valuable polymeric materials based on standard conventional monomeric building blocks, such as block or gradient copolymers, and narrow-polydispersity functional polymers, with narrower functionality distributions, that can be used in film-forming or cross-linking compositions, such as coating compositions, adhesives and printing ink formulations.
In the field of “living” radical polymerizations (polymerizations under radical conditions where termination processes of growing radicals are reversible, so that all initiated chains can, in principle, continue to grow as long as monomers and radicals are present), three technologies have been studied in depth:                nitroxide mediated polymerizations (NMP), which are based on homolytic scission of the covalent bond formed between a polymer radical and a nitroxide stable radical;        atom transfer radical polymerization (ATRP), in which a halogen (usually Cl or Br) atom is reversibly transferred between a Cu(I/II) complex and a polymer radical, in a process that encompasses a redox cycle;        reversible addition fragmentation chain transfer (RAFT), which is a process wherein dithioester end groups can be rapidly and reversibly transferred between polymeric radicals.        
For all these systems, the living character was proven by a typically linear development of molecular weight with conversion, low polydispersities obtained in polymerizations, and the preparation of block copolymers by sequential addition of different monomers.
ATRP and RAFT especially have possibilities for numerous applications with respect to the types of monomers used. In ATRP, however, acidic conditions and acidic monomers create problems, while the necessary removal of the Cu-amine complexes from the polymers is an expensive and thus unwanted step. In RAFT, these limitations do not occur, but this method suffers in that the polymer chains formed contain dithioester end groups, which are strong chromophores. The chromophores can be destroyed by reaction with a nucleophile, but this nucleophile is not always compatible with the functional groups that may be present in the chains. Such chromophore destruction comes at the expense of an extra reaction step and leads to low-molecular weight products in the polymer, which may be difficult to remove. Moreover, the dithioester mediating compounds are expensive.
In general, RAFT based on dithio-ester compounds is assumed to be favourable in the rate of end group transfer (more transfer events, lower polydispersity, better control). A lower polydispersibility is desirable when the polymer is used in e.g. film-forming or cross-linking compositions. A narrow molecular weight distribution in such applications leads to a good balance of pot life and curing speed, viscosity and network properties. Further, in the art it is assumed that the RAFT process is applicable to a broad range of monomers.
NMP has the disadvantage that expensive nitroxides are needed if the process is to be used at acceptably low temperatures for acrylates, whereas methacrylates have intrinsic problems due to the fact that disproportionation may occur as a side reaction.
Another type of living radical polymerization process is degenerative iodine transfer (DIT) (see for instance Journal of Physical Organic Chemistry, Vol. 8, 306–315 (1995) and Macromolecules, 28, 8051–8056 (1995)). In this process, as in the RAFT process, an iodine end group can transfer from one polymer to a radical end group of another polymer present in the polymerization system, the iodine atom playing a similar role to that of the dithioester group in RAFT, the terminal iodine and the radical function exchanging between two polymer chains. A chain transfer agent with an iodine atom is used as entry in the DIT process. Conventional iodine chain transfer agents include alkyl or perfluoroalkyl iodides.
In Macromolecules, 28, 8051–8056 (1995), Macromolecules, 33(9), 3485 (2000), Macromolecules, 32(22), 7354 (1999), Macromolecules, 31(9), 2809 (1998) the DIT process was described for application with styrene monomers, for which it appears to work moderately well (polydispersities typically 1.5), and for application with halogenated vinyl monomers like vinyl chloride and fluorinated vinyl species.
DIT was attempted for acrylates, but the results showed less control (broader polydispersities>2, indicating a low rate of transfer between acrylate chains) (see: Macromolecules, 28, 8051–8056 (1995)). DIT processes with acrylates have been disclosed in a few documents: however, these aim at obtaining iodine-functional chains instead of aiming at actual living polymerizations (see for instance U.S. Pat. No. 6,143,848). DIT processes to prepare block copolymers of acrylates and styrene have been reported in EP 947527, Macromolecules, 28, 2093 (1995), and Macromol. Rapid Commun., 2000, 21(13), 921.
A few attempts to obtain a degenerative iodine transfer process with methacrylates have been reported, but all of these suggest that methacrylate monomers cannot be used in an acceptable manner in conventional DIT processes using the conventional iodine-functional CTAs, which are applied in the DIT process for e.g. styrene (see for instance Macromolecules, 28, 8051 (1995)). High-molecular weight material is formed at the start of the polymerization, leading to high polydispersities, without the typical linear development of molecular weight with conversion, which is characteristic for a polymerization with a living character, being observed. Another comparison of DIT processes with styrene, acrylate, and methacrylate monomers was disclosed in B. Klumperman at the UNESCO School & South African IUPAC conference on Macromolecular and Materials Science, 29–31 Mar. 1999 and 10–12 Apr. 2000.
Here it is also concluded that DIT is not a suitable process for the polymerization of a mixture of monomers containing a large amount of methacrylate monomers. Therefore, it has been assumed that DIT is not suitable for polymerizing methacrylates in a controlled manner, to obtain the benefits that can be obtained in a “controlled (living) radical polymerization” process in the art. In U.S. Pat. No. 5,439,980 this is confirmed in Comparative example 2, where it was found that when MMA alone is used with a fluorinated alkyliodide, only a homopolymer of MMA is produced and a block polymer with an iodide-functional perfluoropolyether is not produced.
Since methacrylates are a very important class of monomers for many applications, with high- and low-polarity candidates and various functional side groups available, with high-Tg backbones with good chemical durability being formed, the inability to use methacrylates is a serious problem in DIT technology, and a need exists for an effective DIT method allowing the production of polymers based on methacrylates with structures that benefit from the advantages of the living nature of the polymerization process.
It is an object of the present invention to obtain a DIT process that can be used for (co)polymerizing methacrylate monomers as in a living radical polymerization process. According to the present invention, it was found that the DIT process can be performed in a very favourable way using methacrylates, if the proper start-up process is chosen. We have found that the DIT process can be adapted for the polymerization of compositions of predominantly methacrylate monomers, to find a high transfer rate of iodine atoms between methacrylate chain ends, leading to better control than a DIT process using styrenic or acrylate monomers, to low polydispersities closely resembling those of ATRP or RAFT processes, molecular weights increasing with conversion, and the opportunity to prepare well defined block and gradient copolymers.